Ethylene vinyl alcohol copolymer (EVOH) films are excellent barriers to oxygen, carbon dioxide and nitrogen. They are also effective barriers against odors and the loss of flavor. EVOH resins, are moisture sensitive and the barrier properties are reduced in the presence of high humidity. Polypropylene offers excellent moisture barrier properties, together with good strength properties and a high use temperature. When EVOH resins are encapsulated by layers of polypropylene, they ar protected from moisture and therefore retain their barrier characteristics.
The biaxial orientation of EVOH films enhances their gas barrier properties as well as reduces their susceptibility to moisture. The biaxial orientation of polypropylene increases its stiffness and enhances both its optical and other physical properties such as tensile strength, tear strength, and other mechanical properties.
Biaxially oriented coextruded multilayer films in which an EVOH layer is sandwiched between two polyolefin layers, particularly polypropylene layers are described in U.S. Pat. No. 4,561,920 which is incorporated herein by reference. The composite is oriented a maximum of four times in the machine direction (MD) and seven times in the transverse direction (TD).
In accordance with this invention mulilayer films of the same or similar components are successfully oriented to a greater extent than in U.S. Pat. No. 4,561,920 and the benefits of improved film quality associated with higher degrees of orientation are obtained.